When a motor such as a permanent magnet synchronous motor is started, the difference between a magnetic-pole position and the control phase of a rotor may generate a shock at the start or rotate the rotor in the direction opposite to the instructed direction. A conventional motor control apparatus performs an initial magnetic-pole position estimating process to estimate an initial magnetic-pole position of a rotor using the salient pole characteristic and the magnetic saturation characteristic of a motor when the motor is started.
In such an initial magnetic-pole position estimating process, after the magnetic-pole position of the rotor is tentatively estimated, the magnetic-pole position of the rotor is definitively estimated as an initial magnetic-pole position on the basis of the polarity (the north pole, the south pole) at the magnetic-pole position that is tentatively estimated. This tentative estimation is performed by, for example, applying a high-frequency voltage to the motor and estimating the magnetic-pole position of the rotor on the basis of the high-frequency current that is caused by the salient pole characteristic of the motor due to the high-frequency voltage to flow in the motor.
The above-described definitive estimation is performed by, for example, applying a pulse voltage varying between positive and negative voltages to the motor to magnetically saturate the magnetic flux in the d-axis of the motor, detecting the difference in change of the current flowing into the motor to determine the polarity at the magnetic-pole position, and correcting the magnetic-pole position that is tentatively estimated based on the polarity thus identified.
While this initial magnetic-pole position estimating process is a technique based on the premise that the motor is stopped, a technique is proposed that makes it possible to estimate the initial magnetic-pole position of the rotor even when the rotor of the motor is rotating due to free running or drag by a load (e.g., see Japanese Patent Application Laid-open No. 2011-193726). Such a technique prevents the occurrence of overcurrent by performing application of a high-frequency voltage and application of a pulse voltage at the same time, and enabling control of the fundamental wave current.
However, when performing current control while applying the pulse voltage to perform the polarity determination, the difference in current change due to the magnetic saturation on the d-axis magnetic flux becomes smaller because of the effect of the current control, whereby the accuracy of the polarity determination is reduced. This requires use of a bandpass filter or increased amplitude of the pulse voltage to avoid the effect of the band of the pulse voltage on the current control, thereby complicating the circuit and highly increasing the voltage of the pulse voltage.